Function stepCron

packages/effect/src/Cron.ts:476–634

(cron: Cron, startFrom: DateTime.DateTime.Input | undefined, direction: "next" | "prev")

/** @internal */
const stepCron = (cron: Cron, startFrom: DateTime.DateTime.Input | undefined, direction: "next" | "prev"): Date => {
  const tz = Option.getOrUndefined(cron.tz)
  const zoned = dateTime.unsafeMakeZoned(startFrom ?? new Date(), {
    timeZone: tz
  })

  const prev = direction === "prev"
  const tick = prev ? -1 : 1
  const table = cron[direction]
  const boundary = prev ? cron.last : cron.first

  const needsStep = prev
    ? (next: number, current: number) => next < current
    : (next: number, current: number) => next > current

  const utc = tz !== undefined && dateTime.isTimeZoneNamed(tz) && tz.id === "UTC"
  const adjustDst = utc ? constVoid : (current: Date) => {
    const adjusted = dateTime.unsafeMakeZoned(current, {
      timeZone: zoned.zone,
      adjustForTimeZone: true,
      disambiguation: prev ? "later" : undefined
    }).pipe(dateTime.toDate)

    const drift = current.getTime() - adjusted.getTime()
    if (prev ? drift !== 0 : drift > 0) {
      current.setTime(adjusted.getTime())
    }
  }

  const result = dateTime.mutate(zoned, (current) => {
    current.setUTCSeconds(current.getUTCSeconds() + tick, 0)

    for (let i = 0; i < 10_000; i++) {
      if (cron.seconds.size !== 0) {
        const currentSecond = current.getUTCSeconds()
        const nextSecond = table.second[currentSecond]
        if (nextSecond === undefined) {
          current.setUTCMinutes(current.getUTCMinutes() + tick, boundary.second)
          adjustDst(current)
          continue
        }
        if (needsStep(nextSecond, currentSecond)) {
          current.setUTCSeconds(nextSecond)
          adjustDst(current)
          continue
        }
      }

      if (cron.minutes.size !== 0) {
        const currentMinute = current.getUTCMinutes()
        const nextMinute = table.minute[currentMinute]
        if (nextMinute === undefined) {
          current.setUTCHours(current.getUTCHours() + tick, boundary.minute, boundary.second)
          adjustDst(current)
          continue
        }
        if (needsStep(nextMinute, currentMinute)) {
          current.setUTCMinutes(nextMinute, boundary.second)